Computer Networks with Internet TechnologyWilliam Stallings

Chapter 14Data Link Control

Flow Control• Ensuring the sending entity does not

overwhelm the receiving entity—Preventing buffer overflow

• Transmission time—Time taken to emit all bits into medium

• Propagation time—Time for a bit to traverse the link

Figure 14.1 Model of Frame Transmission

Stop and Wait• Source transmits frame• Destination receives frame and replies

with acknowledgement• Source waits for ACK before sending next

frame• Destination can stop flow by not send ACK• Works well for a few large frames

Fragmentation• Large block of data may be split into small

frames—Limited buffer size—Errors detected sooner (when whole frame

received)—On error, retransmission of smaller frames is

needed—Prevents one station occupying medium for

long periods

• Stop and wait becomes inadequate

Sliding Windows Flow Control• Allow multiple frames to be in transit• Receiver has buffer W long• Transmitter can send up to W frames

without ACK• Each frame is numbered• ACK includes number of next frame

expected• Sequence number bounded by size of field

(k)—Frames are numbered modulo 2k

Figure 14.2 Sliding Window Depiction

Figure 14.3 Example of a Sliding Window Protocol

Sliding Window Enhancements• Receiver can acknowledge frames without

permitting further transmission (Receive Not Ready)

• Must send a normal acknowledge to resume

• If duplex, use piggybacking—If no data to send, use acknowledgement

frame—If data but no acknowledgement to send, send

last acknowledgement number again, or have ACK valid flag (TCP)

Error Control• Error detection• Error correction• Single bit errors• Burst errors

Error Detection• Additional bits added by transmitter for

error detection code• Parity

—Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones

—Even number of bit errors goes undetected

Figure 14.4 Error Detection Process

Cyclic Redundancy Check• For a block of k bits transmitter generates

n bit sequence• Transmit k+n bits which is exactly

divisible by some number• Receive divides frame by that number

—If no remainder, assume no error—For math, see Stallings chapter 7

Error Control• Detection and correction of errors• Lost frames• Damaged frames• Automatic repeat request

—Error detection—Positive acknowledgment—Retransmission after timeout—Negative acknowledgement and

retransmission

Automatic Repeat Request (ARQ)• Stop and wait• Go back N• Selective reject (selective retransmission)

Stop and Wait• Source transmits single frame• Wait for ACK• If received frame damaged, discard it

—Transmitter has timeout—If no ACK within timeout, retransmit

• If ACK damaged,transmitter will not recognize it—Transmitter will retransmit—Receive gets two copies of frame—Use ACK0 and ACK1

Figure 14.5 Stop-and-Wait ARQ

Stop and Wait - Pros and Cons• Simple• Inefficient

Go Back N (1)• Based on sliding window• If no error, ACK as usual with next frame

expected• Use window to control number of

outstanding frames• If error, reply with rejection

—Discard that frame and all future frames until error frame received correctly

—Transmitter must go back and retransmit that frame and all subsequent frames

Go Back N - Damaged Frame• Receiver detects error in frame i• Receiver sends rejection-i• Transmitter gets rejection-i• Transmitter retransmits frame i and all

subsequent

Go Back N - Lost Frame (1)• Frame i lost• Transmitter sends i+1• Receiver gets frame i+1 out of sequence• Receiver send reject i• Transmitter goes back to frame i and

retransmits

Go Back N - Lost Frame (2)• Frame i lost and no additional frame sent• Receiver gets nothing and returns neither

acknowledgement nor rejection• Transmitter times out and sends

acknowledgement frame with P bit set to 1

• Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i )

• Transmitter then retransmits frame i

Go Back N - Damaged Acknowledgement• Receiver gets frame i and send

acknowledgement (i+1) which is lost• Acknowledgements are cumulative, so

next acknowledgement (i+n) may arrive before transmitter times out on frame i

• If transmitter times out, it sends acknowledgement with P bit set as before

• This can be repeated a number of times before a reset procedure is initiated

Go Back N - Damaged Rejection• As for lost frame (2)

Figure 14.6 Go-Back-N ARQ Protocol

High Level Data Link Control• HDLC• ISO 33009, ISO 4335

HDLC Station Types• Primary station

—Controls operation of link—Frames issued are called commands—Maintains separate logical link to each

secondary station

• Secondary station—Under control of primary station—Frames issued called responses

• Combined station—May issue commands and responses

HDLC Link Configurations• Unbalanced

—One primary and one or more secondary stations

—Supports full duplex and half duplex

• Balanced—Two combined stations—Supports full duplex and half duplex

HDLC Transfer Modes Normal Response Mode NRM• Unbalanced configuration• Primary initiates transfer to secondary• Secondary may only transmit data in

response to command from primary• Used on multi-drop lines• Host computer as primary• Terminals as secondary

HDLC Transfer Modes Asynchronous Balanced Mode ABM• Balanced configuration• Either station may initiate transmission

without receiving permission• Most widely used• No polling overhead

HDLC Transfer Modes Asynchronous Response Mode ARM• Unbalanced configuration• Secondary may initiate transmission

without permission form primary• Primary responsible for line• Rarely used

Frame Structure• Synchronous transmission• All transmissions in frames• Single frame format for all data and

control exchanges

Figure 14.7 HDLC Frame Structure

Flag Fields• Delimit frame at both ends• 01111110• May close one frame and open another• Receiver hunts for flag sequence to synchronize• Bit stuffing used to avoid confusion with data

containing 01111110—0 inserted after every sequence of five 1s—If receiver detects five 1s it checks next bit—If 0, it is deleted—If 1 and seventh bit is 0, accept as flag—If sixth and seventh bits 1, sender is indicating abort

Figure 14.8 Bit Stuffing

Address Field• Identifies secondary station that sent or will receive

frame• Usually 8 bits long• May be extended to multiples of 7 bits

—LSB of each octet indicates that it is the last octet (1) or not (0)

• All ones (11111111) is broadcast

Control Field• Different for different frame type

—Information - data to be transmitted to user (next layer up)

• Flow and error control piggybacked on information frames

—Supervisory - ARQ when piggyback not used—Unnumbered - supplementary link control

• First one or two bits of control filed identify frame type

• Remaining bits explained later

Control Field Diagram

Poll/Final Bit• Use depends on context• Command frame

—P bit—1 to solicit (poll) response from peer

• Response frame—F bit—1 indicates response to soliciting command

Information Field• Only in information and some

unnumbered frames• Must contain integral number of octets• Variable length

Frame Check Sequence Field• FCS• Error detection• 16 bit CRC• Optional 32 bit CRC

HDLC Operation• Exchange of information, supervisory and

unnumbered frames• Three phases

—Initialization—Data transfer—Disconnect

Initialization• Initialization requested by either side• Issue one of six set-mode commands

—Signals other side that initialization is requested—Specifies mode (NRM, ABM, ARM)—Specifies 3- or 7-bit sequence numbers

• If request accepted HDLC module on other side transmits unnumbered acknowledged (UA) frame

• If request rejected, disconnected mode (DM) sent

Data Transfer• Both sides may begin to send user data in I-frames

— Starting with sequence number 0— N(S) and N(R) fields of I-frame are sequence numbers— Sequential, modulo 8 or 128, (3- or 7-bit) — In N(S)— N(R) acknowledgment for I-frames received— I-frame expected next

• S-frames also used for flow and error control— Receive ready (RR) frame acknowledges last I-frame received

• Indicating next I-frame expected• Used when no reverse data

— Receive not ready (RNR) acknowledges, but also asks peer to suspend transmission of I-frames

— When ready, send RR to restart— REJ initiates go-back-N ARQ

• Indicates last I-frame received has been rejected• Retransmission beginning with N(R) required

— Selective reject (SREJ) requests retransmission of single frame

Disconnect• Either HDLC module can initiate disconnect• Own initiative

—Fault

• At request of higher-layer user• Send disconnect (DISC) frame• Remote entity must accept by replying with

UA —Inform layer 3 user connection terminated

• Outstanding unacknowledged I-frames may be lost—Recovery left to higher layers

Figure 14.9 Examples of HDLC Operation

Required Reading• Stallings chapter 14• Web sites on HDLC